It is crucial to determine which model of audio drivers are being tested. Then, disconnect the outside speakers from the two terminals on the back of the amplifier.
Put a DC OHM meter across the terminals.
A reading of 9 - 10 OHMs means the drivers and wiring are probably good. If higher, then one or more of the drivers could be open, or a connection is corroded. If lower, one or more drivers or the wiring may be shorted.